H3O+, also known as the hydronium ion, is a positively charged ion formed when an H2O molecule gains an extra hydrogen ion (H+). This process occurs in aqueous solutions, particularly when acids dissolve in water. The presence of H3O+ is crucial for understanding acidity, as it directly relates to the concentration of hydrogen ions in a solution. In chemistry, the concentration of H3O+ determines the pH level of a solution. A higher concentration of H3O+ results in a lower pH, indicating a more acidic solution. Conversely, a lower concentration leads to a higher pH, which signifies a more basic or alkaline solution.